Method of making chrome-tanning baths



Patented Mar. 7, 1950 UNITED STATES PATENT OFFICE METHOD OF MAKING CHROME-TAN NIN G BATHS No Drawing. Application April 8, 1947, Serial No. 740,276

2 Claims.

This invention relates to a combination chrome tannage of leather and to corresponding chrometanning solutions and the preparation thereof.

It has heretofore been proposed to prepare chrome tan liquors by reducing alkali metal dichromate solutions with liquid sulphur dioxide (U. S. Patent 1,983,733 to Frederick W. Binns). It has also been proposed to prepare chrome tan liquors by reducing an alkali metal dichromate solution by any suitable procedure (but preferably with sulphur dioxide), and then adding an organic acid to the reduced solution in certain proportions and in order to develop a preferred pH value therein, preferably between 2.8 and 3.0, and then to tan hides with the resulting tanning liquor, in the usual manner or after pickling and draining (U. S. Patent 2,382,204 to Hiram B. Channon) In accordance with the present invention it has been discovered by us that a chrome tanning liqnor is advantageously prepared from two separately reduced solutions of alkali metal dichromate and that such a tanning liquor is especially eiiective not only for use in a combination tannage of the leather and in the tanning action which it has upon the hides but in the improved quality of the tanned leather which may be obtained as a result of its application.

In such procedure, one of the solutions may be prepared by controllably reducing the alkali dichromate solution with sulphur dioxide to the monobasic chromium salt, CIOHSOIL Such a reaction may be effected substantially quantitatively and therefore the condition and concentration of the reduced chromium is definitely controllable and dependable. The resulting solution may be modified to any predetermined degree by the addition of inorganic or organic buffering agents, such as alum and sodium hydroxide, organic acids, such as acetic, formic or oxalic acids or salts of organic acids such as sodium formate or sodium phthalate. The pH value of the resulting solution may thus be adjusted to the desired range of acidity or basicity for the particular tanning operations in question.

The second reduced alkali metal dichromate solution is preferably prepared by reducing an aqueous solution of an alkali metal dichromate with a sugar, in the presence of a mineral acid. In this procedure, if theoretical proportions of sugar are employed for the reduction, (i. e., 17.2% by weight of sugar or in terms of its glucose equivalent CtHmOs upon the weight of the anhydrous dichromate) it is found that a certain quantity of the alkali metal dichromate remains unreduced in the solution to the extent of about 5% or more. This proportion of residual unreduced dichromate may be decreased by employing excess quantities of sugar over and above the theoretical quantity, for example, 25% to 50%, or more. f

It is found that the use of an excess of sugar results in the formation of organic by-products, as Well as the presence of some unreacted sugar in the resulting solution. However, it is found that these tendencies may be controlled to some extent by conducting the reaction in the presence of predetermined proportions of a mineral acid. It is therefore preferable for the purposes of the present invention to prepare the sugar-reduced alkali metal dichromate solution either by using substantially theoretical proportions of sugar in the reduction, even though it does leave some of the alkali metal dichromate unreduced, 'or' of using an excess of sugar, in a concentrated'dh chromate solution, and in either case conducting the reduction in the presence of a mineral acid such as sulphuric acid. Under these conditions the presence of unreduced dichromate is rela tively controllable, or may be practically completely eliminated and the amounts of free organic acids formed may also be predetermined.

Accordingly, the first alkali metal dichromate solution may be reduced with sulphur dioxide to the degree desired, i. e., to a basicity corresponding to that represented by the formula CIOHSO4', by using an equivalent of sulphur dioxide, and the acidity or pH value of the resulting solution may'then be regulated by the addition of an alkali, a bufier salt, or the like.

On the other hand the second solution may be prepared by reducing a concentrated alkali metal.

dichromate solution with an excess of sugar, in the presence of a free mineral acid solution, thereby effecting substantially complete reduction of the dichromate to CIOHSO i, or to a basicity as high as C12(OH)4SO4, and with the formation of free organic acids, and organic byproducts other than CO2 but in relatively controlled proportions.

The reduced solutions as thus prepared are found to be capable of supplementing each other in respect of their properties and tanning action. The sugar reduced solution, in order to be reduced in its dichromate component, becomes acidic with both the mineral acid and organic acid by-products, some of which are not desirable to introduce into the hides as well as other acidic and non-acidic organic by-products, which are beneficial. On the other hand, the sulphur dioxide reduced dichromate solution may be carried to complete reduction and then brought to any degree of acidity or pH value required as by adding an inorganic buffering salt such as aluminum sulphate and/or caustic soda during or after the reducing reaction.

The sulphur dioxide reduced dichromate solution may also contain some free or reactive S02 therein, if desired, to advantage.

By mixing appropriate proportions of the reduced dichromate solutions as thus separately obtained, (e. g., in substantially equal amounts with reference to the original dichromate used and/or with reference to the reduction reaction products in each) a combined chrome tan liquor is obtained which is found in actual experience to be Surprisingly effective in its application to hides. It not only permeates the hide structure but also the individual lumens of the fibers of which the hides are composed. It effects the filling of the hide throughout in such a comi plete way as to produce a finished leather which is marked not only by its surface texture but by a plumping of the inner portions of the hide to a degree which is not attained by either type of tanning solution when used by itself.

One outstanding feature of the combination chrome-tannning liquor as prepared by such procedure is that a control over the reduction of the original dichromate is effected in the preparation of each of the separately prepared solutions, and also by subsequent admixture of the two, preparatory to the tanning operation. In one or the other of these solutions, some unreduced dichromate may advantageously be still present when these solutions are combined. This minor amount of unreduced dichromate is sub- 'ject to subsequent reduction in the combined chrome-tanning liquor, either by reaction with residual sulphur dioxide, if any, and/or by still further promoted reduction and reaction with the sugar or the organic by-products of the sugar solution.

Such subsequent reduction of the partially reduced dichromate, in the combined tannage liquor, may also effect complex ion formation of the resulting chromic ions with those of the chrome salts already present in one of the separately prepared solutions or subsequently formed. For example, if the first dichromate solution is only partially reduced to a basic chromium salt CrOHSOmnd its pH value controlled, as by addition of suitable buffering agents thereto, and then added to the second, more completely reduced dichromate solution or one having a lower 'basioity of the chromic salt (or a lower pH value) but competent to reduce the first, the dichromate content of the latter will be reduced and rendered capable of entering into reaction with the chromium ions thereof, to form double compounds containing complex chromium ions. Such complex ions are found to be of especially appropriate characteristics in the tanning action of the resulting tanning liquor in the hides and in the resulting leather.

A representative example of the practical application of the invention to chrome tannage, on an industrial scale, will be described.

For this purpose, 1200 gallons of Water may be run into a suitable tank, usually lead lined, but which may be of other sufiiciently chemically resistant material. To this, 4,800 pounds of sodium dichromate (that is, commercial crystalline sodium dichromate, as Na2C1zO1.2H2O) are added and dissolved, preferably with agitation by a stream of compressed air. When solution of the dichromate is complete, water is added to make the total volume of solution up to 1680 gallons, with continued agitation. The air stream is then shut off and a stream of liquid sulphur dioxide is introduced into the solution and dispersed near the bottom of the tank, preferably in fine streams, to avoid vaporization and escape of the sulphur dioxide gas into the air. The reaction of the sulphur dioxide, in reducing the dichromate, heats up the reaction mixture. When the temperature reaches 140 F., the sulphur dioxide stream is shut 01? and the charge is again agitated with a stream of compressed air.

Caustic soda solution, preferably at a concen' tration of one-half pound per gallon, is then added, to provide 240 pounds NaOH, also with air agitation to effect uniform mixing.

The air is then shut off and liquid sulphur dioxide again introduced into the bottom of the solution, until the temperature of the mixture reaches 180 F. The sulphur dioxide is then shut off, 480 pounds of alum (aluminum sulphate crystals) are added, with agitation by air, until dissolved, and the air stream is again shut off.

Sulphur dioxide is again introduced into the bottom of the charge, which is kept at 180 F., or below by circulating cooling water through a closed coil immersed therein, until the required reduction is complete. The further addition of sulphur dioxide may be thus conducted until a total of 3096 pounds of liquid sulphur dioxide have been added to the charge. During this latter period of the reduction, water ma be added to the solution, so long as sufficient space is left in the tank for the ultimate volume of liquid sulphur dioxide which is to be used. The sulphur dioxide stream is then shut off.

The final volume of the charge described is brought up to 2400 gallons by the addition of water, agitated thoroughly with compressed air and allowed to stand for a sufficient period to become completely reacted and to cool off. It is then again made up to 2400 gallons, with additional water and agitation, and is ready for use after a thorough final agitation, in the preparation of the combination chrome tannage liquor. It may contain a proportion of unreduced sodium dichromate.

The second reduced alkali metal dichromate solution may be prepared by first dissolving 4800 pounds of sodium dichromate in 600 to 1,000 gallons of water, followed by the addition of 1200 pounds of granulated sugar. The mixture is thoroughly agitated to effect complete and uniform solution.

Sulphuric acid (5040 pounds calculated as 100% HzSO-i) is then allowed to trickle into the solution of dichromate and sugar. The reaction results in the evolution of heat, eventually bringing the charge to the boiling point. The rate and degree of heating may be controlled by the rate and aggregate amount of sulphuric acid added, at any given time, so that boiling over of the charge may be prevented and uniform reaction and boiling are maintained.

After cooling the reaction mixture substantially to room temperature, the reduced solution is made up to 2400 gallons, by the addition of water.

As an alternative of the foregoing procedure, the preparation of the second reduced dichromate solution may be effected by preparing the water solution of alkali metal dichromate, as described, then adding the required amount of sulphuric acid and agitating until uniformity is established.

The sugar is then separately dissolved in a convenient volume of water, and the sugar solution allowed to trickle into the acid-alkali metal dichromate solution. The resulting heat evolved is then controlled by the rate and amount of addition of sugar solution, from time to time, instead of by the addition of sulphuric acid to the solution of sugar and dichromate.

There is some difierence in the effects of the two procedures. In the first instance the reducing reaction is effected in a concentrated sugar and dichromate solution, but dilute with respect to the successive additions of the acid component, leading to a tendency to form organic by-products to a greater degree, such as organic acids and the like. In the second instance, the reducing reaction is effected in a concentrated acid and dichromate solution, but dilute with respect to the successive additions of sugar, upon which the reducing action is dependent. The latter procei dure therefore leads to a somewhat slower and more uniform progress of the reaction and to less formation of organic by-products such as organic acids and the like. But in both cases, the concentration of dichromate is relatively high and the reduction thereof to the chromic salt is substantially complete. The difference between the two solutions is therefore primarily in respect of the oxidation by-products of the sugar,

whether to carbon dioxide and a residue of relatively undecomposed sugar, or to a series of intermediate reaction products resulting from intermediate degrees of various oxidation reactions thereon.

By way of comparison it may be noted that in preparing solutions reduced with a sufficient proportion of sugar to efiect complete reduction of Pounds Sodium dichromate 4000 H2SO4 (calculated as 100%) 2800 Glucose 2100 (The glucose is in the form of a syrup of about 50% concentration of glucose.)

The basicity of the reduced dichromate solution thus prepared is about 48% by the official method of calculation of the American Leather Chemists Association.

In the preparation of sugar reduced dichromate solutions, using an amount of sugar in excess of the theoretical equivalent required, organic by-products other than CO2 are produced which remain in solution. These organic by-products are variable both in kind and in the proportions formed, in respect of any compound or group of compounds in question. Thus, although certain organic acids may be thus formed which are desirable, other undesirable organic acids may be formed or excessive amounts of the desirable organic acids may be introduced into the tanning liquor, beyond those amounts or concentrations which are *found to be desirable in accordance with the disclosure of the patent to Channon, above referred to. In such cases, upon admixture of the sugar reduced dichromate solution witha sulphur-dioxide reduced dichromate solution, the chrome tannage value of the solution need not be decreased but may be increased while such excess concentration of organic acids in the resulting tanning liquor is reduced and their further formation is suppressed.

Accordingly, the separately prepared solutions of reduced alkali metal dichromate, as above ob-' tained, are then mixed, in varying proportions as required, for example, in the instant case preferably in approximately equal proportions, relative to the original dichromate equivalents used or to their total chromium contents.

In the procedures as thus conducted, it will be observed that the first solution is reduced with somewhat less than the amount of sulphur dioxide which would be required (3096 lbs.) to reduce the amount of dichromate used completely to monobasic chromic sulphate CrOHSOa due to evaporation losses and dilution of the unreduced dichromate. Such amount of sulphur dioxide therefore leaves some unreduced sodium dichromate, and the subsequent addition of caustic soda and aluminum sulphate produces a solution having a basicity of about 42%. This compares with a solution of dichromate which if it were completely reduced to monobasic chromic sulphate, CrOI-ISO4, would have a basicity of 33 without the addition of any buffering agent.

The sugar reduced dichromate solution, on the other hand, may be substantially completely reduced in respect of the dichromate, to the monobasic chromic sulphate, and contains an excess of sugar (512 lbs.) and consequently an excess of organic compounds, such as free acetic, formic and oxalic acids and other by-products of the oxidation reaction upon the sugar. Likewise more (304 pounds) than a reaction equivalent of free sulphuric acid may be employed in its preparation.

Upon mixing the two solutions, and in subsequently tanning hides with the combination chrome tannage solution thus formed, the free organic acids and by-products of the sugar reduced dichromate solution are in a condition to complete the reduction of dichromate in the sulphur dioxide reduced dichromate solution and promote penetrative and tanning qualities of the whole through its organic acids and the reducing by-products of the sugar, in such proportions as it is added thereto.

The hides to be tanned, usually after bathing, pickling, and allowing to drain, are then introduced into a sufficient volume of the chrome tannage liquor thus obtained, in a wheel or rotat able drum with the usual addition of common salt, and drummed for a suitable length of time to efieot saturation. Sodium bicarbonate solution is added from time to time, during the drumming period, after which the hides are removed, horsed up and allowed to drain in the usual way.

It is found that hides tanned by this method of chrome tannage procedure, and with the mixture of separately prepared reduced alkali metal dichromate solutions, as above described, provide leathers which are characterized by a more uniform and complete tannage than when tanned with singly reduced alkali metal dichromate solutions of the prior art, and have a greater degree of plumpness or fullness throughout the leather. Furthermore, when tacked up, after tanning, and allowed to dry, they manifest substantially no tendency to draw or shrink when the tacks are removed.butiretairrztheirrshape and dimensions throughout:

It iszobserved as :a part oflthe-present' invention that, in the reduction of dichromate with sugars,-i\vhile the required amount of sugar may beadded inorder to efiect a sufficient degree of reduction, the amount of sulfuric-acid added may be varied inyterms of the degreeof basicity of the resultingsolution desired. Thus the greater.

the percentage ofbasicity (by the officialmethod offithe American Leather Chemists Association) the less the amount of the sulphuric acid will be added,'and conversely the lower the basicity desired the greater the-amount of sulphuric acid will beadde'd. But the amounts of sulphuricacid added iwillsnot be directlyproportioned to the degree of basicity required. In general, the more sugar'there is added, and the more of the sulphuricacid there'is added to a given amount of alkali dichromate, the greater will'be the formation of organic acidsas by-products of the reducing reaction upon the-dichromate, that is, instead of carbondioxide, free carbon, or run reduced sugar. These free organic acids therefore tend to increase-the acidity and also to reduce the partially'reduced dichromate solution and the basicity' of the-resulting chromic salt. This may be offset or compensated for by using a lesser amount of sulphuric acid. In preparing the tanning liquors above described, corresponding to basicity of 42%, approximately 6% more than the theoretical quantityor" sulphuric acid was added but buffering agents such ascaustic soda and alumrwere added whichv raised its basicity-to 4.2-, accordingly, while in the preparation of aftannin'g liquor of: other basicities, more or less than thetheoretical proportion of sulphuric acid 'may 'be used, toproduce tanning liquors of the appropriate basicities and pH values."-

Thus, in both of the sugar-reduced dichromate solutions above described the pH values are approximately 2.40.

The sulphur dioxide-reduced dichromate solution, separately prepared, had a pH value of approximately 2.50.

Upon mixingythese solutions, therefore there is not'a; very pronounced Ivariation'in the pH values of the mixtures produced, but the sulphur dioxide reduced dichromate' solution tends to dominate in this respect as will appear from the following table:

(1) 100% sugar reduced (lichromnte solution pH 2.40

3) S: reduced dichromatc solution pH e is?useassassinat (5) 100% S02 reduced dichromate solution pH 2.50

In this case the S02 reduced dichromate solution was slightly less acidic it will be observed. But it can be regulated in this respect more directly than the sugar reduced dichromate solution, above or below the pH value of the sugar reduced dichromate solution, and. thus determine the pH value of the resulting combination tanning liquor prepared by admixture of the two separately prepared solutions.

It is further observed that while the formation and presence of a certain limited proportion or concentration of the organic acids is desirable in the-reduced dichromate solution and in the use of such solution for tannage purposes, excessive proportions of free acids or free organic acids arernot desirable.v Correlative to this observationisthe .discovery that in sugar reduced dichromatesolutions, the excess of the sugar r carbon dioxide) and which is decomposed in the reaction, leads to the formation of organic acids to the extent of about thereof, and approximately 20% of such decomposed sugar leads to the formation ofintermediate non-acidic organic by-products of undetermined composition, but which are beneficial in the use of such solutions for tanning.

Of the organic acids thusformed in the sugar reduced dichromate solution, and especially oxalic, formic and acetic acids, the proportions and concentrations of oxalic acid and-formic acid are not detrimental in the tanning liquor. But acetic acid tends to be formed in greater propor tions and amounts than is desirable because with increasing proportions of acetic acid a rapidly increasing de-tanning action sets in which is definitely detrimental in the tanning action upon hides and in the tanned leather produced.

Accordingly,'by the admixture of SOs-reduced dichromate solutions and sugar reduced dichromate solutions in appropriate proportions these advantages are retained and the disadvantages of excess acid-formation (and more particularly acetic acid) as well as'of any unreduced dichrom'ate, etc., are controlled. Moreover, the organic acids of the sugar reduced solution serve to promote the ultimate reduction of all. unreduced dichromate, and arein turn converted into the alkali metal and chromic salts of such acids, which also enter into complex chromium ion formation with the chromic salts of the inorganic acid, namely CrOI-ISO4..

While in the examples above described the sulphur dioxide reduced dichromate solution is but partially reduced, and hence contains some unreduced'alkali metal dichromate and the sugarreduced solution is completely reduced and con tains an excess of sugar, or oxidation products thereof and free organic acids, it is to be understood that both' solutions maybe reduced substantially completely to the monobasic chromium sulphate, and the pH values and organic acid components adjusted, by proportioning the ratios in which they are combined independently of their dichromate equivalents; and it is also to be understood that'the SOs-reduced alkali dichromate solution may be completely reduced to the monobasic chromic salt and that the sugarreduced alkali dichromate solution may be but partially reduced to basic chromic salt but will contain sugar, organic oxidation products of the sugar and organic acids whichwill be in reduced condition and competent to complete the reduction of such residual dichromate upon admixture with the SOz-reduced solution, especially if the latter contains or is made to contain a slight excess of free sulphur dioxide dissolved therein.

It should be understood that the present disclosure is for the purpose of illustration only and that this invention includes all modifications and equivalents which fall within the scope of the appended claims.

Weclaim:

l. The method of preparing tanning liquors which comprises the steps of reducing a solution of an alkali metal dichrcmate with sulfur dioxide, reducing another solution .of an alkali metal dichromate with atleast a stoichiometric excess of sugar in the presenceof .sulfuric acid, and mixing the two solutions. inthe. proportions of 25 to 75% S02 reduced dichromate to '75 to 25% REFERENCES CITED sugar reduced dichromate.

2. The method of preparing tanning liquors g z g f gfi are of record m the which comprises the steps of reducing a solution of an alkali metal dichromate with sulfur dioxide, .3 UN D STATES PATENTS reducing another solution of an alkali metal di- Number Name Date chromate with at least about a 25% stoichiometric 2 332 204 Channon Aug 14 1945 excess of sugar in the presence of sulfuric acid, n

and mixing the two solutions in the proportions of OTHER REFERENCES 25 t0 S02 reduced dichromate t0 t0 The Chrome Tanning Process W Merry sugar-reduced dichromate- Chemical Pub. Co. Inc. New York, 1936, pages 14,

FREDERICK W. BINNS. 30 and 31 HIRAM BINGHAM CHAN NON 

1. THE METHOD OF PREPARING TANNING LIQUORS WHICH COMPRISES THE STEPS OF REDUCING A SOLUTION OF AN ALKALI METAL DICHROMATE WITH SULFUR DIOXIDE, REDUCING ANOTHER SOLUTION OF AN ALKALI METAL DICHROMATE WITH AT LEAST A STOICHIOMETRIC EXCESS OF SUGAR IN THE PRESENCE OF SULFURIC ACID, AND MIXING THE TWO SOLUTIONS IN THE PROPORTIONS OF 25 TO 75% SO2 REDUCED DICHROMATE TO 75 TO 25% SUGAR REDUCED DICHROMATE. 